1. Field of the Invention
This invention relates to a small brushless dc motor unit having armature windings supported by a stator, a permanent-magnet rotor and a solid-state switching circuit that electronically commutates dc power to energize the armature windings. The timings for the commutation switchings are determined by the angular position of the rotor that is detected by utilizing the back emf voltages in the armature windings induced by the revolving rotor.
2. Description of the Prior Art
A conventional brushless dc motor unit of this kind will be explained in reference to FIGS. 10 and 11.
Referring to FIG. 10, a conventional brushless motor 101 has Y-connected 3-phase armature windings 102-1, 102-2, 102-3 disposed in a stator (not shown) and a permanent-magnet rotor 103. A dc voltage VM is provided to a solid-state electronic commutation circuit 105 from a dc power supply 110. The commutated output voltages are individually provided to the armature windings 102-1, 102-2 and 102-3 from 3-phase bridge-connected output terminals 106-1, 106-2 and 106-3, respectively, of the commutation circuit 105 to drive the motor 101. The commutation circuit 105 consists of six solid-state switching elements (or, transistors) 107-1, 107-2, 107-3, 107-4, 107-5 and 107-6, having respective control terminals that are individually connected to six switching control outputs of a control unit 108.
The switching elements 107-1.about.6 are turned on and off by switching control signals transmitted from the control unit 108 at specific rotor angles. The switching sequence is arranged to cause the armature windings to produce a rotating magnetic flux in the air gap that interacts with the flux produced by permanent magnets on the rotor 103 so as to rotate the rotor in synchronism with the rotating magnetic field.
The induced terminal voltages of the 3-phase armature windings 102-1, 102-2 and 102-3 are provided to 90.degree. phase delay filter circuits 111-1, 111-2 and 111-3, respectively, so that the phase angle of each voltage is delayed by 90.degree. thereby. The reason for the delay angle being 90.degree. for the conventional brushless dc motor will be discussed later. The phase delay filter circuits 111-1, 111-2 and 111-3 consist of resistors R14, R15 and R16, respectively, connected in series to the respective armature winding terminals, and capacitors C6, C7 and C8, respectively, connected in parallel as shown. The 90.degree. phase-delayed output voltages Fu, Fv and Fw, of the filter circuits 111-1, 111-2 and 111-3, respectively, are provided to the positive input terminals of voltage comparators 112-1, 112-2 and 112-3, respectively. To the negative input terminals of the voltage comparators 112-1.about.3 is commonly provided a midpoint voltage VN. The midpoint voltage VN is obtained by summing the phase-delayed voltages Fu, Fv and Fw through resistors R17, R18 and R19, respectively.
FIG. 11 is a waveform chart for explaining the function of the control circuits of the conventional brushless dc motor unit shown in FIG. 10. FIG. 11(A) shows a waveform of the terminal voltage Vu of the winding 102-1. FIG. 11(B) shows a waveform of the 90.degree. phase-delayed output voltage Fu of the phase delay filter circuit 111-1 and the midpoint voltage VN. FIG. 11(C) shows a waveform of an output voltage Cu of the voltage comparator 112-1. FIG. 11(D) shows "ON" state timings of the six solid-state switching elements 107-1(U.sup.+), 107-2(V.sup.+), 107-3(W.sup.+), 107-4(U.sup.-), 107-5(V.sup.-) and 107-6(W.sup.-).
The terminal voltage Vu is a trapezoidal wave that has spike voltages Vsp at the ends of the "ON" states of the corresponding switching elements 107-1(U.sup.+) and 107-4(U.sup.-), as shown in FIG. 11(A) along with FIG. 11(D). The other terminal voltages Vv, Vw have a like waveform and spikes, though there exists a 120.degree. phase shifting one another among the three-phase terminal voltages Vu, Vv and Vw.
The spike voltages Vsp appear when the currents to the windings 102-1, 102-2 and 102-3 are interrupted by the commutation circuit 105. The phase-delayed output voltages Fu, Fv and Fw of the filter circuits 111-1, 111-2 and 111-3, respectively, are compared to the midpoint voltage VN by the voltage comparators 112-1, 112-2 and 112-3, respectively, which output voltages Cu, Cv and Cw, respectively. The output voltage Cu has a rectangular waveform that rises or falls at the moments the voltage Fu becomes even with the midpoint voltage VN, as shown in FIG. 11(C) along with FIG. 11(B). The other output voltages Cv, Cw have a like waveform, but with shifted phase angles.
At the rising edge 115 of the output voltage Cu, a control unit 108 generates control signals that are individually provided to the control terminals of the switching elements 107-5 and 107-6 so that the control signals cause the switching element 107-5(V.sup.-) to be turned off and the switching element 107-6(W.sup.-) to be turned on, as shown in FIG. 11(D) along with FIG. 11(C). On the other hand, at the falling edge 116 of the output voltage Cu, the control unit 108 generates control signals that are individually provided to the control terminals of the switching elements 107-2 and 107-3 so that the control signals cause the switching element 107-2(V.sup.+) to be turned off and the switching element 107-3(W.sup.+) to be turned on. In other words, the turn-on timings for the W-phase switching elements and turn-off timings of the V-phase switching elements are derived from the terminal voltage of the U-phase winding 102-1.
Control signals are also similarly generated in the control unit 108 in reference to the output voltages Cv (V-phase) and Cw (W-phase) and individually provided to the corresponding switching elements. In this manner, the armature windings 102-1, 102-2 and 102-3 are provided with 3-phase dc voltages through the commutation circuit 105, so that the motor is driven.